Variable resistance control

ABSTRACT

A ceramic substrate supports a plurality of moisture resistant cermet film resistive paths having a uniform temperature coefficient of resistance, a center collector, a contactor wipingly engaging the collector and a resistive path, and a contactor driver having a stub shaft with an enlarged portion thereof bearing against the substrate. Terminations on the substrate comprise conductive pads disposed adjacent to passageways in the substrate and a protective layer of solder deposited over such pads. In a variable voltage divider embodiment, a surge limiting resistive path is provided between the collector and an output termination to reduce the magnitude of high frequency, high potential pulses applied to the output termination of the divider.

United States Patent [72] inventors Arthur L. Rozeml:

Wayne A. Burden: Wilbert H. Budd, all of Elkart, Ind. [211 App]. No, 782,031 [22) Filed Dec. 9, 1968 [45] Patented June 15, 1971 [73] Assignee CTS Corporation Elklurt, Ind.

[541 VARIABLE RESISTANCE CONTROL 12 Claims, 4 Drawing Figs.

[52] US. Cl. 338/48, 338/174 [51] Int. Cl. 1101c 9/02 [50] Field Search 338/162, 163,164, 165, 166,167, 168, 169, 170, 171, 172, 173,174,175, 306, 308, 309,l22,123,124, 48,

[56] References Cited UNITED STATES PATENTS 3,421,133 1/1969 Van Benthuysen et al. 338/174 Assistanl Examiner-D. A. Tone Artorneys- John J. Gaydos and Ralph E. Krisher, Jr.

ABSTRACT: A ceramic substrate supports a plurality of moisture resistant cermet film resistive paths having a uniform temperature coefficient of resistance, a center collector, a contactor wipingly engaging the collector and a resistive path, and a contactor driver having a stub shaft with an enlarged portion thereof bearing against the substrate. Terminations on the substrate comprise conductive pads disposed adjacent to passageways in the substrate and a protective layer of solder deposited over such pads. In a variable voltage divider embodiment, a surge limiting resistive path is provided between the collector and an output termination to reduce the magnitude of high frequency, high potential pulses applied to the output termination of the divider.

PATENTEUJummn $585,559

FIGURE- 2 INVENTORS ARTHUR L. ROZEMA WAYNE A. BARDEN WILBERT BY WQ'T FIGURE- VARIABLE RESISTANCE CONTROL This invention relates generally to variable resistance controls and, more particularly, to controls that are particularly suitable for use as variable voltage dividers.

Variable voltage dividers usually comprise two resistive paths, a collector, a contactor wipingly engaging one of the resistive paths and the collector, a driver for changing the position of the contactor relative to the resistive paths, and a plurality of termination means for connecting the voltage divider with the circuitry of suitable electronic apparatus. It will be appreciated that only a fraction of the total cost of such apparatus is attributable to variable voltage dividers 1 used therein. Yet, it will also be appreciated that malfunctioning of such voltage dividers can markedly affect the overall operation of such apparatus. In view of these facts, it would be desirable to provide an improved voltage divider that would be reliable while operating in changing ambient conditions and that could be readily assembled with electronic apparatus without being damaged.

In recent years, as the degree of sophistication of electronic equipment has increased, it has become increasingly important for voltage dividers to exhibit stable operating characteristics that are independent of the humidity or temperature of the environment surrounding such voltage dividers. In practical terms, this means that it is important that the voltage appearing at the output termination of the voltage divider be substantially constant regardless of changes in ambient conditions. In variable voltage dividers wherein the resistive paths are a cermet film, i.e., a film formed of a vitreous matrix and a conductive fraction normally comprising one or more metals or compounds of metal, a solderable conductive pad is often provided at the terminal end of the resistive paths to facilitate the completion of electrical connections with the divider. In the past, terminals suitable for connection to an external circuit have been soldered to the conductive pads. However, this technique has created series solder connections between the conductive pads and the terminal points of the circuit connected thereto and has increased the number of possible cold solder joints between the conductive pads and the circuit connected thereto. Accordingly, it would be desirable to provide terminations on a variable voltage divider wherein the number of series solder connections between conductive pads and an electrical circuit are minimized. It would also be desirable to protect the conductive pads from contamination that would degrade the solderability of the pads and to provide means for reducing the sensitivity of the conductive pads to overexposure to molten solder since such overexposure can result in the leaching of noble metals from the conductive pads by the molten solder.

In one specific application, variable voltage dividers are used to supply a constant voltage to the focus electrode of cathode-ray tubes. In this application, one end (the input end) of a voltage divider is connected to a relatively high potential power supply, the other end (ground end) of the divider is connected to a low potential or ground, and the output termination of the divider is connected to the focus electrode of the tube. In such applications, when a cathode-ray tube arcs back, i.e., when an arc occurs between the high voltage anode and the relatively lower voltage focus grid within the tube, a high voltage, high frequency pulse is impressed on the output termination of the voltage divider and means must be provided to prevent damage to the components of the power supply connected to the input termination of the divider. Therefore, it would be desirable to incorporate such means as an integral part of variable voltage dividers used in cathoderay tube applications.

Accordingly, it is an object of the present invention to provide a new and improved variable voltage divider wherein a plurality of film-type cermet resistive paths each have the same temperature coefficient of resistance. Another object of the present invention is to provide a new and improved variable voltage divider wherein for a given input voltage, the output voltage is constant notwithstanding changes in ambient temperature. A further object of the present invention is to provide means integral with a variable voltage divider for reducing the magnitude of high frequency, high voltage pulses applied to the output termination of the voltage divider. An additional object of the present invention is to provide an improved variable voltage divider wherein conductive pads on the divider are protected from contamination and manifest reduced effective sensitivity to molten solder. A still further object of the present invention is to provide an improved electrical component having a ceramic substrate and a molded contactor driver having a portion thereof bearing against the substrate. Still an additional object of the present invention is to provide a new and improved electrical component com-prising a plurality of electronic devices and resistive paths having the same temperature coefficient of resistance. Further objects and advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

The present invention is concerned with a ceramic substrate supporting a plurality of film-type resistive paths on a surface thereof, a center collector, a contactor driver having a stub shaft projecting through an aperture in the substrate and having an end of the stub shaft enlarged to bear directly against a surface of the substrate and secure assembly of the driver with the substrate, a conductive contactor wipingly engaging one'of the resistive paths and the center collector and constrained to rotate with the contactor driver, and a conductive pad disposed at an end of each of the resistive paths. In a specific embodiment of the invention, a surge limiting resistive path connects the collector to a conductive pad carried on the substrate. In still another specific embodiment, one or more active electronic devices, i.e., one or more solid-state devices such as transistors or integrated circuits, and one or more passive electronic devices, i.e., one or more capacitors, resistors, or inductors are supported on the surface of the substrate and interconnected with each other. by conductive paths on the substrate. In each of the embodiments, cermet films resistive exploded isometric view of the variable voltage divider of FIG.

1; FIG. 3 is a sectional view taken along the line Ill-Ill of FIG. 1; and FIG. 4 is a plan view of a substrate used in another em bodiment of the invention.

Referring now to FIGS. 1-3, there is illustrated a variable voltage divider generally indicated by the numeral 10, comprising a substrate 11 formed of a ceramic material such as alumina supporting resistance means in the form of a first resistive path 12, a second resistive path 13, and a third resistive path 14; a conductive collector l6; and a plurality of conductive terminations which include conductive terminal pads 17-19 that comprise a quantum of solderable precious metal with a protective layer of solder deposited thereon. Hereinafter, the conductive pad 17 may be referred to as the input termination, the conductive pad 18 will be referred to as the low potential or grounded termination, and the conductive pad 19 will be referred to as the output termination. Although any one of several commercially available materials may be used to form the resistive paths l2-14, it is preferable that the resistive paths be formed from a cermet resistive material, i.e., a material such as the material disclosed in the Faber et al. US. Pat. No. 3,304,199 which, after firing, forms a moisture resistant film comprised of a glassy phase with a conductive fraction dispersed therein.

The conductive pads l7--19 are preferably comprised of a film of palladium-gold or other suitable solderable conductive material that will form a good electrical connection with the material used to make the resistivepaths. This same material may also be used to form the collector 16 and may be applied inany suitable manner, e.g., by screening. By making the resistive paths 12- 14 from a common cermet formulation and by applying these paths to a common surface 11a of the substrate, variations occurring in the processing of the resistive paths such as variations in the pressures used while applying the resistive paths to the surface of the substrate or variations in firing time or temperature will have substantially an identical effect on each of the resistive paths. Accordingly, the resistive paths will each have the same temperature coefficient of resistance which is defined as the number of parts change, per million, of the ohmic value of a resistor per degree centigrade. Therefore, for any given setting of the voltage divider 10, the ratio of the ohmic value of the resistive path between the input and output termination to the ohmic value between the center collector and the low potential termination of the divider will remain constant for any ambient temperature. It will be understood that this result is attained because changes in ambient conditions that cause a given percentage change in ohmic value in one portion of the voltage divider will cause the same percentage change in the ohmic value of all other portions of the voltage divider.

A preferred form of adjusting means for repeatedly changing the setting of the voltage divider 10, i.e., changing such setting as many as 300 times, comprises a conductive contactor 21 trapped and resiliently compressed between the planar surface 11a of the substrate and a contactor driver 22 with which the contactor is constrained to rotate because of the interengagement of a pair of cars 21a and 21b on the contactor with a pair of notches 23 formed in the contactor driver. Furthermore, the driver forms a protective cover for the contactor when assembled with the substrate. In order to secure assembly of the contactor driver and conductive contactor with the substrate 11, an alignment member illustrated as a stub shaft 26 is formed integrally with the contactor driver and aligns the contactor driver with the substrate by projecting through an aperture 27 in the substrate 11. As best illustrated in FIG. 3 and end 26a of the stub shaft 26 is enlarged by heat swaging and forms a bearing 26b against a surface 11b of the substrate spaced from and parallel to the surface 11 a. The contactor driver 22 also includes a knurled adjusting shaft 27 that facilitates manual adjustment of the divider. In order to space the driver 22 from the surface 11a of the substrate and to provide for smooth adjustment of the control, a bearing 28 is provided on the face of the driver 22. Rotational movement of the driver is limited by the stop arm 220 that travels in an interference path with abutment surfaces 11b and 11c of the substrate. The conductive pads 17-19 are located near the peripheral edges of the substrate adjacent to notches or passageways 11d, 11e, 11] in the substrate 11. These passageways form part ofthe terminations of the voltage divider and are located for cooperation with terminals carried by a circuit board or other not shown apparatus with which the voltage divider 10 is to be used. The conductive pads 17-19 are formed of suitable solderable material such as palladium-gold and, although such material is not normally considered to be readily oxidizable the surface of the conductive pads 17-19 may becomes sufficiently oxidized or contaminated during storage to reduce the solderability thereof. This contamination may result when the conductive pads are exposed to an atmosphere containing chlorides, sulfides, dust, or oil vapors that are deposited on the pads. Because of this problem, the conductive pads are protected by depositing thereon a protective layer of solder as shown at 31-33 in the drawings. The protective layers of solder are preferably deposited by dipping the terminations in a solder bath or by any other consistently controllable process. in addition toprotecting the surface of conductive pads 17-19, the layers 31- 33 of solder reduce the effective sensitivity of the conductive pads to exposure to molten solder by reducing the amount of time that the conductive pads must be subsequently exposed to molten solder while being manually connected to external circuitry. To connect the terminations of the voltage divider 10 to an external circuit, it is only necessary to apply heat to the terminations until the layers 31-33 of solder begin to flow. This is particularly advantageous because the noble metals used in the conductive pads are leachable by molten solder and it is relatively easy to overexpose the conductive pads 17-19 to molten solder, i.e., expose the pads 17-19 to molten solder until the noble metals are leached from the pads. Somewhat surprisingly, the protective layers of solder 31-33 have solved this leaching problem. Although the exact mechanism by which this solution is attained is not precisely understood, it is believed that a very thin layer of the solder coating 31-33 adjacent to the conductive pad surfaces either remains solidified or only momentarily becomes molten while connections are being made to an external circuit. Thus, the sensitivity of the conductive pads 17-19 to molten solder during the making of such connections is effectively reduced.

The voltage divider 10 is particularly suitable for use in the focus circuit of a cathode-ray tube and, in a typical color television picture tube application, the input of the voltage divider 10 is connected to a DC voltage of approximately 6,000 volts, the low potential termination is connected to ground potential and the output is connected to the focus terminal of the picture tube. The tube is then focused by adjusting the contactor driver 22 until a voltage of from about 4,000 to about 6,000 volts is applied to the focus electrode of the tube. Since all of the resistive paths on the surface 11a of the base 11 have the same temperature coefficients of resistance, a constant voltage will be applied to the tube even though the ambient temperature may change. As will be understood, arcs intermittently occur between the high voltage anode of cathode-ray tubes and the focus grid in the tubes. When this condition occurs, a damaging, high frequency, pulse of as much as 30,000 volts may be impressed upon the power supply connected to the input of the voltage divider. in order to prevent damage to the power supply under such conditions, we have provided a surge limiting resistor 14 which is effective to create a voltage drop when such arcing occurs and limit the magnitude of the pulse appearing at the input of the voltage divider. In one actual reduction to practice of the invention, the base 11 was approximately 2% inches long, about 1% inches high overall, and about seven thirty-seconds inches thick. Measurements of the ohmic value between the conductive test point 35 and the ground termination indicated that the resistance of resistive path 12 was 30 megohms and other measurements indicated that the resistance of resistive paths 13 and 14 were 20 megohms and 6 megohms, respectively.

Now having reference to FIG. 4, it will be appreciated that the embodiment comprising substrate 40 is not, strictly speaking, a voltage divider since a plurality of electronic devices and conductive areas are supported on a surface 40a of the substrate 40. For example, a plurality of palladium-gold conductive areas 41-49, passive devices in the form of fixed resistive paths 50-52, a pair of capacitor 53-54, and an inductance coil 55 are supported on the substrate in addition to a diode 57 and quadrac 58. The center collector 59 is connected to the conductive path 47 and, when a contactor 21 and contactor driver 22 are assembled with the substrate 40, the contactor wipingly engages the resistive path 51 and collector 59. The resistive paths 50-52 of FIG. 4 have substantially uniform temperature coefficients of resistance. A pair of terminations 61, 62 comprise portions of the conductive areas 41 and 43 adjacent to passageway notches 63, 64 and are covered by protective layers of solder 65, 66.

While there has been illustrated and described what is at present considered to be two preferred embodiments of the present invention, it will be appreciated that numerous changes and modifications are likely to occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention.

What we claim as new and desire to be secured by Letters Patent of the United States is:

l. A variable voltage divider comprising an electrically nonconductive substrate having at least one substantially planar surface, a plurality of conductive terminations supported on said surface, resistance means comprising a first resistive path supported on said surface and connected to a first one of the conductive terminations, a conductive collector supported on said surface, adjusting means supported for movement relative to the substrate, and a conductive contactor constrained to move with said adjusting means and wipingly engage the first resistive path and the conductive collector, said resistance means further comprising a second resistive path supported on said surface and interconnecting the conductive collector and a second one of the conductive terminations thereby to provide a voltage drop between the collector and the second one of the conductive tenninations when a potential is applied thereacross.

2. The variable voltage divider of claim 1, wherein the conductive terminations comprise a quantum of solderable precious metal and a protective layer of solder covering said precious metal.

3. The variable voltage divider of claim 1, wherein the conductive collector comprises a quantum of precious metal and the adjusting means provides a protective cover overlying the contactor.

4. The variable voltage divider of claim 1, wherein the substrate comprises a ceramic member having a second surface spaced from and substantially parallel to said planar surface,

the terminations include passageways for receiving and cooperating with terminals carried by a circuit board, and a layer of solder is deposited adjacent to each of the passageways, said layers of solder facilitating the making of electrical connections with such tenninals.

5. The variable voltage divider of claim I, wherein the nonconductive substrate comprises a ceramic body having a second surface substantially parallel to said planar surface, the first resistive path is a cennet film supported on said planar surface, an aperture is formed in said substrate communicating with said planar and said second surfaces, and the adjusting means supported for movement relative to the substrate comprises a contactor driver disposed adjacent to the planar surface of the substrate and a stub shaft integral with the driver and disposed in said aperture, said stub shaft having an enlarged portion bearing against the second surface and providing a bearing for the shaft that prevents removal of the driver from the ceramic body.

6. The variable voltage divider of claim 5, wherein said driver overlies the conductive contactor and the conductive contactor is resiliently compressed between the driver and the '6 substrate.

7. The variable voltage divider of claim 6, wherein the substrate is provided with a pair of abutment surfaces and the driver includes a stop arm engageable with said abutment surfaces.

8. The variable voltage divider of claim 7, wherein the second resistive path is supported on a first portion of the substrate, the first resistive path is supported on a second portion of the substrate.

9. An electrical component comprising an electrically nonconductive substrate having a pair of spaced substantially planar surfaces, a plurality of terminations supported on a first one of the surfaces, a collector supported on the substrate and connected to a first of the terminations, a first resistive path supported on said one of the surfaces, one end of the first resistive path being connected to a second of the terminations, a second resistive path supported on said first one of the surfaces, one end of the second resistive path being connected to a third of the terminations, the other ends of the first resistive path and the second resistive path being connected together, a conductive contactor engaging the collector and the first resistance path intermediate the ends thereof, said first and second resistive paths each having substantially the same temperature coefficient of resistance for maintaining the ratio of the ohmic value between the first and the second of the termrnations and between the first and the third of the terminations constant for any ambient temperature, an aperture formed in said substrate communicating with said pair of surfaces, adjusting means comprising a contactor driver disposed adjacent to the first surface of the substrate and an alignment member connected to the contactor driver and disposed in the aperture, the conductive contactor being constrained to move with the driver and wipingly engage the first resistive path and the collector, the driver overlying the conductive contact, the conductive contactor being resiliently compressed between the driver and the substrate, said alignment member including a bearing for holding the adjusting means and contactor in assembled relationship with the substrate.

10. The component of claim 9, wherein a third resistive path is supported on said first one of the surfaces and is connected between the collector and the first of the terminations.

11. The component of claim 9, wherein the terminations comprise passageways for accommodating terminals connected to an external circuit.

12. The component of claim 9, wherein the terminations comprise a quantum of precious metal and protective layer of solder deposited thereover. 

1. A variable voltage divider comprising an electrically nonconductive substrate having at least one substantially planar surface, a plurality of conductive terminations supported on said surface, resistance means comprising a first resistive path supported on said surface and connected to a first one of the conductive terminations, a conductive collector supported on said surface, adjusting means supported for movement relative to the substrate, and a conductive contactor constrained to move with said adjusting means and wipingly engage the first resistive path and the conductive collector, said resistance means further comprising a second resistive path supported on said surface and interconnecting the conductive collector and a second one of the conductive terminations thereby to provide a voltage drop between the collector and the second one of the conductive terminations when a potential is applied thereacross.
 2. The variable voltage divider of claim 1, wherein the conductive terminations comprise a quantum of solderable precious metal and a protective layer of solder covering said precious Metal.
 3. The variable voltage divider of claim 1, wherein the conductive collector comprises a quantum of precious metal and the adjusting means provides a protective cover overlying the contactor.
 4. The variable voltage divider of claim 1, wherein the substrate comprises a ceramic member having a second surface spaced from and substantially parallel to said planar surface, the terminations include passageways for receiving and cooperating with terminals carried by a circuit board, and a layer of solder is deposited adjacent to each of the passageways, said layers of solder facilitating the making of electrical connections with such terminals.
 5. The variable voltage divider of claim 1, wherein the nonconductive substrate comprises a ceramic body having a second surface substantially parallel to said planar surface, the first resistive path is a cermet film supported on said planar surface, an aperture is formed in said substrate communicating with said planar and said second surfaces, and the adjusting means supported for movement relative to the substrate comprises a contactor driver disposed adjacent to the planar surface of the substrate and a stub shaft integral with the driver and disposed in said aperture, said stub shaft having an enlarged portion bearing against the second surface and providing a bearing for the shaft that prevents removal of the driver from the ceramic body.
 6. The variable voltage divider of claim 5, wherein said driver overlies the conductive contactor and the conductive contactor is resiliently compressed between the driver and the substrate.
 7. The variable voltage divider of claim 6, wherein the substrate is provided with a pair of abutment surfaces and the driver includes a stop arm engageable with said abutment surfaces.
 8. The variable voltage divider of claim 7, wherein the second resistive path is supported on a first portion of the substrate, the first resistive path is supported on a second portion of the substrate.
 9. An electrical component comprising an electrically nonconductive substrate having a pair of spaced substantially planar surfaces, a plurality of terminations supported on a first one of the surfaces, a collector supported on the substrate and connected to a first of the terminations, a first resistive path supported on said one of the surfaces, one end of the first resistive path being connected to a second of the terminations, a second resistive path supported on said first one of the surfaces, one end of the second resistive path being connected to a third of the terminations, the other ends of the first resistive path and the second resistive path being connected together, a conductive contactor engaging the collector and the first resistance path intermediate the ends thereof, said first and second resistive paths each having substantially the same temperature coefficient of resistance for maintaining the ratio of the ohmic value between the first and the second of the terminations and between the first and the third of the terminations constant for any ambient temperature, an aperture formed in said substrate communicating with said pair of surfaces, adjusting means comprising a contactor driver disposed adjacent to the first surface of the substrate and an alignment member connected to the contactor driver and disposed in the aperture, the conductive contactor being constrained to move with the driver and wipingly engage the first resistive path and the collector, the driver overlying the conductive contact, the conductive contactor being resiliently compressed between the driver and the substrate, said alignment member including a bearing for holding the adjusting means and contactor in assembled relationship with the substrate.
 10. The component of claim 9, wherein a third resistive path is supported on said first one of the surfaces and is connected between the collector and the first of the terminations.
 11. The component of claim 9, wherein the terminations comprise passagewaYs for accommodating terminals connected to an external circuit.
 12. The component of claim 9, wherein the terminations comprise a quantum of precious metal and protective layer of solder deposited thereover. 